The foundations of the treatment of the short period were laid by Marshall, and it is his discussion of the concept in Principles and his conception of quasi-rent which, together with the additions of later economists, provide the material upon which the author elaborates here.Definitions; the supply schedule of a single firm under perfect competition; the effects of a loss; supply schedule of an industry under perfect competition; shape of the prime cost curves; further consideration of a state of perfect competition; imperfection of the market; equilibrium in an imperfect market; mathematical treatment of the equilibrium of an industry in an imperfect market; changes in prime cost and in hours in an imperfect market; the business man is not a true economic man; the corporate instinct; de-rating.